Common C# errors developers still make in 2026 (and how to fix them)

Working with C# on a daily basis means you will run into compiler errors that can be frustrating, especially when the message isn’t immediately clear.

In this post, I’ve collected some of the most common C# errors I encountered in real-world development, along with practical fixes.

1. Inconsistent accessibility

The error message goes like Inconsistent accessibility: base class '...' is less accessible than class '...', which means that you are trying to expose a class publicly, but its base class is more restrictive (e.g., internal or private).

For example:

internal class BaseClass {}

public class DerivedClass : BaseClass {}

To fix this, make sure the base class is at least as accessible as the derived class, in our example would be public class BaseClass {}

2. Access modifiers are not allowed on static constructors

The access modifiers are not allowed on static constructors error means that static constructors in C# cannot have access modifiers like public, private, or protected. Static constructors are access-modifier-less by design and are called automatically by the runtime.

For example, the following is incorrect, public static MyClass() {}.

To fix that simply remove the access modifier, static MyClass() {}

3. An object reference is required

The exact error message is An object reference is required for the non-static field, method, or property and it means that you are trying to access a non-static member from a static context.

For example:

class MyClass
{
    public int Value = 10;

    public static void Print()
    {
        Console.WriteLine(Value); // this will throw an error
    }
}

To fix this error, either create an instance, for example:

var obj = new MyClass();
Console.WriteLine(obj.Value);

or make the member static, like public static int Value = 10;

4. The best overloaded method match has some invalid arguments

The exact error message is The best overloaded method match for '...' has some invalid arguments which means that you are calling a method with parameters that do not match any available overload.

For example:

void Print(int value) {}

Print("hello");

In order to fix this error, make sure the argument types match the method signature.

5. Nullable object must have a value

This error means that you are accessing .Value on a nullable type (int?, DateTime?, etc.) that is currently null.

For example:

int? number = null;
Console.WriteLine(number.Value); 

I would suggest you to avoid using .Value unless you are sure the value is not null. Instead, use either number ?? defaultValue or even better the idiomatic Null-Conditional Operator (?.), which is safely accessing members without throwing an exception.

For example: string display = number?.ToString() ?? "N/A";

6. Converting null literal to non-nullable type (CS8600)

Since the introduction of Nullable Reference Types in C# 8.0, the compiler now tracks whether a variable is allowed to be null. You will see this warning (or error) if you try to assign null to a type that wasn’t explicitly marked as nullable.

For example: string name = null; // Warning CS8600.

To fix it, if the variable is allowed to be empty, add a ? to the type declaration, string? name = null; or if the variable should never be null, provide a default.

If you are absolutely certain the value won’t be null at runtime, you can use the null-forgiving operator: string name = GetNameFromDatabase()!;

7. Required member must be initialized

With the introduction of the required modifier in C# 11, you will frequently encounter the error: Required member '...' must be initialized in object initializer or attribute constructor.

For example:

public class User
{
    public required string Username { get; set; }
}

var user = new User(); // Error: Required member 'Username' is not initialized

To fix this, you must use an object initializer:

var user = new User 
{ 
    Username = "DevInAthens" 
};